Rounding Whole Numbers
Why round numbers?
When exact numbers are not needed, numbers are often rounded to give approximate
values and quantities. Rounding also helps us estimate.
Temperature:
Daily temperatures are often rounded to the
nearest degree Celsius.
E.g., 25.7°C is generally reported as 26°C
Money:
People often use rounding and estimating at
a store to make sure they have enough money
to pay for items before they get to the cashier.
Using large numbers:
• Newspapers often round large numbers to take up less
space. They may print $10M rather than $10 490 721.00.
• When adding or subtracting large numbers mentally, it is
easier to round them first!
Think About …
For the next week, find examples of when you use or hear rounded numbers when
talking to friends, family and other people in your community.
Discuss in groups when and how rounding is used in the workplace.
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Rounding Using Number Lines
A number line has numbers placed in order along a straight line.
Number values increase from left to right.
Check out Hints for Rounding for more rounding strategies.
Rounding to the nearest 10
Example
Round the following numbers to the nearest 10:
11
7
19
15
24
11
7
0
15
19
10
24
20
Where are the numbers on the number line?
The numbers 7 and 11 are close to 10 on the number line.
The numbers 19 and 24 are close to 20 on the number line.
The number 15 is exactly in the middle between the 10 and 20.
Rounding to the nearest 10
7 and 11 are both closer to 10 than they are to 0 or 20.
• 7 rounds up to 10
• 11 rounds down to 10
19 and 24 are both closer to 20 than to 0 or 10.
• 19 rounds up to 20
• 24 rounds down to 20
What about 15?
If a number is exactly half way, round to the larger of the two numbers.
• 15 rounds up to 20 because it is exactly half way between 10 and 20.
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Another way to think about rounding is to pull the number line up into a mountain.
5
4
15
6
3
14
7
2
13
9
1
0
•
•
17
12
8
16
18
11
19
10
20
Any number below 5 rolls down the hill to the left and rounds down.
Any number 5 and above rolls down the hill to the right and rounds up.
Rounding to the nearest 100
Example
Round the following numbers to the nearest 100:
110 180 320 250 224
110
180
100
•
•
•
•
•
224
250
200
320
300
110 rounds down to 100
180 rounds up to 200
224 rounds down to 200
320 rounds down to 300
250 rounds up to 300 because it is exactly half way between 200 and 300.
Rounding to the nearest 1000
Example
1200
1000
•
•
•
•
•
Round the following numbers to the nearest 1000:
1200 1800 3100 2450 1500
1500
1800
2450
2000
3100
3000
1200 rounds down to 1000
1800 rounds up to 2000
2450 rounds down to 2000
3100 rounds down to 3000
1500 rounds up to 2000 because it is exactly half way between 1000 and 2000.
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Rounding to larger numbers
When rounding numbers to the nearest 10 000, 100 000, 1 000 000 and beyond, the
same methods are used for rounding to the nearest 10, 100 and 1000.
Practice: Rounding
Use a number line or other method to round each number to the nearest 10.
Q
1. 24
20
20
22
24
26
28
30
2. 55
3. 87
4. 66
5. 193
Use a number line or other method to round each number to the nearest 100.
6. 342
7. 459
8. 112
9. 749
10. 1123
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Use a number line or other method to round each number to the nearest 1000.
11. 4600
12. 9250
13. 2730
14. 3800
15. 23 400
Use a number line or other method to round each number to the nearest 10 000.
16. 64 000
17. 29 050
18. 72 300
19. 83 000
20. 335 400
Use a number line or other method to round each number to the nearest 1 000 000.
21. 3 450 950
22. 7 234 734
23. 9 499 999
24. 1 900 000
25. 6 540 100
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26. Round each number in the chart below to the indicated place value. For each
column, go back to the original number on the left. Two examples are shown for
you.
Round to the nearest
Number
1 000 000
100 000
10 000
1000
100
10
23 549 214
24 000 000
23 500 000
23 550 000
23 549 000
23 549 200
23 549 210
10 000
6000
6400
6390
6387
72
32749
310 056
7 328 074
462
3472
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Practice: Rounding Problems
1. Tommas has two savings accounts. In the first one, he has $324.08 and in
the second he has $896.54. He also has $245.76 in a chequing account.
a. Use rounding to 100 to estimate the amount of money Tommas has all together.
$324.08
$896.54
$245.76
rounds to $300
rounds to $900
rounds to $200
Rounding to 100, Tommas has an estimated $1400 in his three accounts.
b. Calculate the exact amount in his accounts. How close is the estimation to the
real value of his accounts?
c. Discuss these questions with classmates:
- In what situations would Tommas need to know the exact amount
of money in his accounts?
- When do you need exact numbers, and when would estimated
and rounded numbers be appropriate? Think about situations in
your life.
2. After receiving her baby-sitting money, Shelly went
shopping. Shelly had a total of $90.00 in her wallet.
Use rounding and estimation to determine the items
that she can purchase so that she spends close to,
but not more than $90.00.
Create at least two combinations of items.
Sweater – $24.56
Capri Pants – $32.98
Sunglasses – $12.45
Blazer – $33.40
Jeans – $56.98
Watch – $26.99
Hat – $8.99
Shoes – $18.95
T-shirt – $16.78
Boots – $53.59
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3. Taylor’s parents are thinking about buying a recreational trailer to park out at the
lake for the summer. There were 4 trailers in which they were interested. The first
trailer cost $54 753, the second cost $19 403, the third was $48 265 and the fourth
trailer was $32 581. How much is each trailer rounded to the nearest 10 000?
4. Check out the use of large numbers in newspapers, magazines,
tables in atlases and other information sources. Discuss the use of
rounded numbers and exact numbers in the information that you find.
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